Process for extracting and drawing a tow

ABSTRACT

An improved process for extracting impurities from synthetic filaments which comprises passing the filaments in the form of a tow, which is simultaneously being drawn, through a series of extraction baths, wherein a portion of the liquor from any given bath is recycled and sprayed with high impact on the fibrous tow at a point above the surface of the bath.

Unitefi States Patent 1 1 Bowen 1 1 Apr. 3, 1973 s41 PROCESS FOR EXTRACTING AND 2,714,052 7/1955 Wizon et al. ..8/137.5 DRAWING A ow 2,979,767 4/1961 Fry ..264/180 3,158,880 12/1964 Osban ..8/l51 lnvemofi Bram" Bowen, Rlchmond, 3,389,206 /1968 Jamison ..264/184 [73] Assignee: E. I. du Pont de Nemours and Com- 3 252 2212 21323 2 2 2222; 332; wlmmgmn' 3:440:842 4/1969 Pace ..68/62 [22] Filed: Aug. 3, 1971 3,558,765 1/1971 Bruner et al ..264/182 [21] Appl' 168616 Primary Examiner-Jay H. Woo

' Attorney-Gary A. Samuels [52] US. Cl. ..264/210 F, 8/137.5, 8/151,

8/151.2, 28/595, 264/184, 264/206, 264/290 ABSTRACT [51] a "D021 D060 1/00 D:lf3/06 An improved process for extracting impurities from [58] e d o g fi 7 63,? 21 synthetic filaments which comprises passing the fila- 8/ 8/ ments in the form of a tow, which is simultaneously being drawn, through a series of extraction baths, [561 References Cited wherein a portion of the liquor from any given bath is UNITED STATES PATENTS recycled and sprayed with high impact on the fibrous tow at a point above the surface of the bath. 2,155,324 4/1939 Moritz ..264/38 2,521,748 9/1950 Rodgers ..264/38 3 Claims, 2 Drawing Figures PATENTEUAPR3 I973 ATTORNEY PROCESS FOR EXTRACTING AND DRAWING A TOW BACKGROUND OF THE INVENTION I solution of the polymer from which the filaments are to be made through a spinneret, either into a heated chamber where much of the solvent is removed by evaporation, or into a fluid coagulation bath where the polymer is precipitated in fiber form. In either case the resulting filaments usually contain impurities such as residual organic solvent, inorganic salts, etc. which were present in the initial polymer solution. In most cases, it is necessary to reduce or completely remove such impurities from the filaments to improve their mechanical properties, thermal and light stability, decrease stickiness, etc. Thus, it is common to treat the spun filaments by gathering a large number of filaments into a tow which is passed through a series of aqueous extraction baths in order to remove the residual salt and/or solvent, etc.

The effectiveness of such an extraction depends in part on the number of baths, on the length of time the filaments are exposed to each bath, maintaining successive baths at progressively lower concentration of impurities, etc. Higher extraction efficiency becomes both more desirable and more difficult as the filaments are generated at higher velocities and as greater numbers of filaments are'gathered into each tow in order to attain commercially attractive levels of production. Since there is a severe economic penalty incurred for simply adding more and larger extraction baths and/or employing higher consumption of aqueous extractant (cost of providing pure water plus either disposal or further processing of impurity-containing effluent), the art has provided other techniques for improving extrac tion efficiency including counter-current flow of tow and extractant through the baths, mechanical means such as squeeze-rollers (wringers) or deflector bars (U.S. Pat. No. 3,391,672) or stripper bars (U.S. Pat. No. 3,381,506) which minimize undesirable transport of extractant liquid by the tow from bath to bath, and various other mechanical devices such as guides and rollers which alter the cross-sectional shape of the tow in order to redistribute the filaments for improved exposure to the extraction baths. Frequently, these various techniques are employed simultaneously in the pursuit of improved extraction efficiency.

It is also well known in the art to improve the tensile properties of synthetic filaments by drawing as-spun filaments to increase the degree of axial orientation of their polymer molecules. Again, for reasons of production economy, the drawing step is frequently performed on a large number of filaments in the form of a tow. Furthermore, in many cases it is desirable or even essential to perform the drawing operation while the residual solvent concentration in the filaments still exceeds some minimum value.

A'coordingly,-it is advantageous to attempt to perform thedfawing arid the extraction operations simultaneouslyoh the fibrous tows. Unfortunately, not only does the high tension necessarily present on the tow in order to induce drawing of the filaments adversely affect extraction efficiency, but one of the prior art extraction aidsrearranging the transverse shape of the tow-may no longer be employed without upsetting filament-to-filament denier uniformity. A necessary consequence of altering the cross-section shape of the tow is that individual filaments no longer travel precisely controlled path lengths so that individual filament draw ratios become unequal, thus introducing undesirable filament-to-filament denier non-uniformities.

The present invention provides increased efficiency for extraction of a fibrous tow while it is under drawing tension by a procedure which does not introduce nonuniform drawing of the filaments. The process of the present invention also permits the filaments to be moved at high speed through the extraction baths thus permitting increased output of fiber. The process of permitting more efficient use of extraction liquid solvent and reducing the amount of effluent to be disposed and/or processed for recovery.

SUMMARY OF THE INVENTION In a process for simultaneously extracting and drawing a fibrous tow of at least 50,000 total denier wherein the tow is continuously passed through an extraction bath while under sufficient longitudinal tension to produce permanent elongation of the fibers in the tow, the improvement comprising recycling a portion of the extraction bath via a pump providing a pressure of at least 20 psig (1.4 Kgmlcm to form a hydraulic jet directed transversely at the fibrous tow at a point above "the bath surface with sufficient impact to penetrate and traverse the fibrous tow.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a series of extraction baths, some of which are equipped with the recycling means necessary to the process of this invention.

FIG. 2 is a schematic illustration of one extraction bath which depicts a preferred arrangement of spray nozzles used to impinge recycled liquid onto the filament bundle.

DESCRIPTION OF THE INVENTION The process of this invention may be described by referring to the drawings. Thus, in FIG. 1, fibrous tow 10 is metered by rolls 11, driven by means not shown, into a series of bath tanks 12 through 17. The cut-out section in FIG. 1 denotes that several bath tanks have been omitted. The tow l0 follows a serpentine path as it passes around rolls l8 and 19 associated with each bath. Optionally, rolls 18 and 19 may individually be driven by means not shown to provide controlled increments of the draw in each tank. The tow l0 exits tank 17 via draw rolls 24, driven by means not shown, to provide a preselected overall draw ratio determined by the relative surface speeds of rolls 24 to rolls 11.

Generally, fresh extraction liquid is entered into the last bath of the sequence, bath 17, through inlet 25. Standpipes 26 connect adjacent baths to allow extraction liquid to flow from one tank to another in a direction counter to the direction of the movement of tow 10. If desired, the sequence of standpipe interconnections between adjacent tanks may be broken to provide introduction of alternative extractant liquids, or to permit control of extracted impurity concentration, etc.

In FIG. 1, baths 14 and 16 illustrate the improved process of the present invention. For example, a portion of the liquid from bath 14 is removed through pipe 20 to the intake of pump 22 which provides an elevated hydraulic pressure at its output which is transmitted via control valve 27 to spray nozzle 21 where the liquid is impinged on tow prior to its entry into bath 14. Optionally, a portion of the output from pump 22 may be returned directly to bath 14 via control valve 28 to provide improved mixing and circulation of the extraction fluid in bath 14. Similarly, liquid from bath 16 is withdrawn via pump 29 which supplies nozzle 23 with high pressure extraction fluid. One or more (even including all) of the baths may be equipped with the spray extraction aids of this invention.

FIG. 2 indicates a preferred arrangement of spray nozzles according to the process of this'invention. Nozzles 30a and 30b direct liquid spray from opposite sides onto the fibrous tow 10 as it enters the bath, while nozzles 31a and 31b direct spray onto the tow as it leaves the bath. The nozzles are arranged to impinge the extraction fluid on opposite faces of the tow at points which are displaced along the path of the tow to provide maximum opportunity for the spray to flow through the bundle to displace entrained impurityladen extraction fluid. On the contrary, opposed nozzles tend to coact to strip liquid from the surface of the bundle and may therefore be used where this function is desired; however, they do not provide the flow of extraction liquid through the tow required by the process of the present invention.

It is preferred, as indicated in FIG. 2, to spray the tow both before and after it is immersed in the extraction bath; however, the benefits of the present invention are still realized (although usually to a lesser degree) even though only one pair of nozzles 30 and 31 is employed, or even though spray is applied only to one face of the tow, e.g., as by omitting nozzle 30b. It is, of course, possible, and in some cases beneficial, to employ more than two nozzles per station; but when multiple nozzles are employed, they should be arranged such that the several sprays do not interact to impede the flow of extraction fluid through the tow.

Also indicated in FIG. 2 are stripper bars 320 and 3212, which may optionally be used, as disclosed in US. Pat. No. 3,381,506, to minimize undesirable cocurrent transfer of extractant fluid by stripping off surface liquid from the moving tow 10 and returning it back to the same bath. Such bars 32a and 32b are particularly effective and desirable at higher tow velocities. Alternatively, a pair of directly opposed high-impact fluid jets may be employed to provide surface fluid stripping action, as mentioned above.

As will be apparent, multiple tows may be passed through a given bath concurrently by placing them in simply the result of increased exposure time between fiber and extractant fluid may be demonstrated by maintaining constant the geometry of the system and velocity of the tow while altering only the hydrostatic pressure of the extractant fluid recirculated to the.

spray nozzles. Only by exceeding a certain threshold pressure are the improved results of the present invention obtained. It is postulated that the interstices between the multitude of filaments comprising these large tows (particularly when the tow is under drawing tension) become filled with relatively stagnant extractant liquid which is transported through the apparatus by the tow, and it is believed that this liquid must be displaced by employing transverse hydraulic jets of sufficient impact to penetrate and traverse the tow in order to provide the significantly increased efficiency of extraction observed in the practice of this invention.

In this specification, extracting and drawing process refers to a process wherein fibrous tow is continuously metered into and immersed in a bath of extractant fluid intowhich are transferred impurities from the yarn such as residual organic solvent, residual inorganic salts, residual monomeric or low molecular weight material, etc., and the fibrous tow is continuously withdrawn from the bath at a rate sufficiently in excess of its input velocity to provide a permanent elongation or draw of the fibrous material. The term fibrous tow refers to a bundle of a large number of substantially parallel, continuous filaments having individual deniers of less than about 100. The product of the number of individual filaments times the denier per filament is the total denier of the tow, which for the purposes of this invention, should exceed about 50,000. Such large tows will frequently take the form of bands, due to contact with guiding rollers, having many filament layers across the width and thickness of the band. The phrase forming a hydraulic jet directed transversely at the fibrous tow means that the recycled extraction bath liquid, under a pressure of at least 20 psig (1.4 Kgm/cm) is forced through an exit orifice to form a fluid stream directed to impinge on the fibrous tow at a substantial angle thereto, e.g., greater than about 30 and preferably i 10. The hydraulic jet should have a sufficient cross-sectional dimension to span the full width of the tow, or alternatively, multiple jets may be employed, each contributing to incremental coverage of the full width of the tow. The phrase sufficient impact to penetrate and traverse the fibrous tow means that the hydraulic jet of recirculated extraction fluid must force its way into and through the fibrous tow, thereby simultaneously displacing the more or less stagnant extractant fluid entrained in the interior of the tow. Obviously, the minimum hydraulic pressure required to provide the necessary impact for tow penetration will be a function of the shape of the jet orifice, the orifice-to-tow distance, the angle at which jet/tow impact occurs, the cross-sectional shape (width and thickness) of the tow, the number and size formed on the side of the tow opposite the-point of jet impingement. The fluid from this bulge, presumably comprising a portion of the impinging hydraulic jet fluid plus the stagnant fluid displaced from the interior of the tow, is allowed to flow back into the bath either by running down the exterior surface of the tow or by being removed with suitable downstream surface fluid stripper devices, e.g., bars or opposed jets. If hydraulic pressures appreciably in excess of the threshold pressure are employed, the jet will penetrate the tow with sufficient residual momentum to fall free from the tow, again providing improved extraction efficiency according to the present invention. However, extremely high hydraulic pressures are less preferred in the present invention in view of the concomitant danger of introducing undesirable filament rearrangements within the tow. The present invention is distinguished from those prior art processes comprising liquid application to fibrous tows via low pressure jets or sprinkling devices in that the present process seeks to displace liquid from the interior of the tow rather than simply applying a treatment fluid which is intended to remain in contact with and be-carried along by the tow.

By the simple expedient of adding readily available equipment to a known apparatus and without any increase in bath size or numbers, the present invention can provide a large decrease in the amount of water needed in the extraction of synthetic filaments and, at

the same time, produce filaments with a much reduced impurity content. 1

In the examples that follow, all weight percentages are based on the weight of the polymer unless otherwise specified.

EXAMPLE I A spinning solution consisting of l9percent based on the weight of the solution, of poly(meta-phenylene isophthalamide) in N,N-dimethylacetamide (abbreviated hereinafter DMAc) that contains 45 percent calcium chloride, based on the weight of the polymer, is passed through holes in spinnerets into a heated spinning cell. The polymer has an inherent viscosity of 1.6, and determined at 25C. using a solution of polymer in N,N-dimethylacetamide containing 5 percent lithium chloride based on the weight of the solution, at a concentration of 0.5 gram of polymer per 100 cc. of solution. The filaments that are formed are converged ata" guide at the bottom of the cell where they are flooded with an aqueous liquid. Filaments from adjacent spinning cells are combined to give a large bundle of filaments referred to as a tow. The towis then drawn and extracted in aqueous baths in a IO-tank apparatus of the type referred to in FIG. 1. The tow contains'55,000 filamentsand has a total denier of 110,000. It has a cross-sectional shape approximately 4 inches X 0.03 inch as it passesthrough the draw/extraction tanks. The tow is metered to the apparatus at a velocityof 180 yards (165 meters) per .minute and drawn at a total draw ratio of 5 to 1. The drawn ratio for eachtank is shown in Table I.

The last seven tanks through which the tow travels are interconnected by standpipes as indicated in FIG. 1, fresh water being fed into the last tank and flowing through the preceding tanks (in a direction counter to the tow travel) via the standpipe interconnections. Each of these seven tanks is equipped with spray nozzles as illustrated in FIG. 2. The four nozzles in each of the tanks are High Impact Low Atomizing Slot Jets, No. H5 1 U 8010 VE'EJET NOZZLES manufactured by the Spraying Systems Company of Bellwood, Illinois and they are supplied with the bath liquid by a CHEM-.

PUMP MODEL GB-3K pump having a 6-inch (21.2- centimeter) impeller (manufactured by the Chempump Division of the Fostoria Corporation of Huntingdon Valley, Pennsylvania) at a pressure of 60 pounds per square inch (4.23 kilograms per square centimeter) gauge which provides jets of fluid with sufficient impact to penetrate and traverse the tow. The nozzles are positioned approximately 2.4 inches (6 centimeters) from the tow. The pipe line is provided with a strainer having a 50-mesh screen to remove foreign material that would otherwise pass to the nozzles.

The process is run both with and without the recycling pumps in operation, and with fresh water supplied at rates of 8 and 13 parts water part of fiber produced, respectively. The equilibrium conditions in the tanks for the two cases are'shown in Table I. Table II shows some of the properties of the filaments that are produced. The recycling method uses over percent less water (8 versus 13 parts of water per part of filaments produced) and produces a much higher extracted product in thatthe filaments contain about 23 percent less salt. (Residual salt is particularly deleterious for such fibers.) A further important advantage is a striking decrease in the number of filament wraps on the processing rolls, presumably arising from the ability of the high impact jets of this invention to entangle and lock in any adventitious broken filament ends so that they are carried through the process by the tow instead of forming deleterious roll wraps.

TABLE I k 1 st 2nd 3rd 5th 6th 7th 8th 9th 10th Draw Ratio 1.60 2.33 1.26 1.007 1.007 1.007 1.007 1.007 1.004 1.007 Temperature, C. 85 85 95 95 95 95 95 98 Bath Concentration, wt. based on total weight- Without Pump:

DMAc 27.3 27.2 23.6 8.1 4.1 2.0 1.3 0.5 0.0 0.5 CaCl2 8.27 6.89 6.18 2.01 0.93 0.43 0.51 0.38 0.61 0.00 With Pump:

DMAc 27.4 28.9 29.0 11.3 4.8 1.8 0.6 0.0 0.3 0.4 CaCh 8.37 7.27 6.51 .86 1.16 0.52 0.47 0.43 0.00 0.00

TABLE II With Pump Without Pump Toughness l .0 l 1.05 Elongation, 30.5 31.6 Chloride content, 0.10 0.13

The tow exiting the final draw/extraction tank is optionally dried and crystallized, e.g. by surface contact with suitable heated rolls, and next has finish applied, e.g. via map surfaced rolls, preferably at an application rate of 0.8 to 1.5 percent by weight. Choice of the preferred finish is governed by the intended ultimate product, e.'g. flock for paper making, crimped staple for textile application, etc.

EXAMPLE II Dry-spun fibers are made from a copolymer of acrylonitrile (AN), methyl acrylate (MA), and styrene sulfonic acid (SSA) with a composition having a mole ratio of ANlMA/SSA of 93.7/6.l/0.2 and containing about 25% N,N-dimethylformamide (DMF) and 40% 11,0 based on the weight of the polymer. These fibers are collected into seven parallel tows, in the form of ribbons, each containing 39,200 filaments of about 8 dpf. These tows are fed at an input speed of 89 yards (81.5 meters) per minute to a IO-tank, extraction-draw machine wherein all 10 tanks are connected. The last tank in the sequence is supplied with water at the rate of 2.3 parts of water per part of fiber produced. The tows are sprayed prior to entry into the lO-tank extraction draw machine with 8 gallons (30.4 liters) per minute of an 11 percent solution of DMF in water. The tows are then passed through feed rolls, where liquid (which contains about l.5 percent DMF) draining from these rolls is collected, and then the tows are drawn 4.5X in the 10 countercurrent fiow bath tanks at 95C. A recycled spray system is used in each of the second, fourth, sixth and eighth tanks. The liquid from the bottom of a given tank is pumped at a pressure of about -25 psig (1.4-1.8 Kgm/cm) at about 20 gallons (76 liters) per minute through 32 nozzles to form high impact jets which penetrate and traverse the tow as it enters and exits from the given tank. The tows, on leaving the tenth tank, are passed through a stuffing box crimper and collected in a can. The DMF content of the drawn, crimped filaments prepared by this method is 0.95 percent compared with a content of 1.45 percent for filaments prepared without sprays.

In a second run, the process is operated the same except that: the number of tows is 10; the filaments per tow is 46,900; the pretreatment spray liquor rate is l2 gallons per minute (35.6 liters per minute); the

recycling sprays are located in tanks two, five and 10; and the recirculating liquor rate is 20 gallons per minute (76 liters per minute) per tank.

Results are shown in Table III.

TABLE [II Fresh Water Supply k Residual DMF Sam le Parts per Part Po ymer ln Drawn Tow No prays 2.3 1.45 Inlet Sprays Only 2.3 1.2 inlet and Recirculating Spray 2.3 1.1 Inlet and Recirculating Spray 1.8 1.15 lnlet and Recirculating Spray L6 1.2

These two runs illustrate the improved extraction efficiency provided by this invention, and indicate the potential for reducing water consumption by its use. Furthermore, an additional advantage observed in both runs is a substantial reduction in the number of fused filaments in the tows while operating the draw/extraction process according to the present invention.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for obvious modifications will occur to those skilled in the art.

m'embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the process for simultaneously extracting and drawing a tow of fibers of at least 50,000 total denier to remove residual spinning solvent and salt wherein the tow is continuously passed through an extraction bath while under sufficient longitudinal tension to produce permanent elongation of the fibers in the tow, the improvement which comprises recycling a portion of the extraction bath via a pump providing a pressure of at least 20 psig to form a hydraulic jet directed transversely at the tow at a point above the bath surface with sufficient impact to penetrate and traverse the tow.

2. The process improvement of claim 1 wherein the hydraulic jet is directed onto the tow in at least two portions, each directed on opposite faces of the tow at points displaced along the path of the tow.

3. The process improvement of claim 2 wherein the I tow is sprayed with the hydraulic jet both as it enters and leaves the bath. 

2. The process improvement of claim 1 wherein the hydraulic jet is directed onto the tow in at least two portions, each directed on opposite faces of the tow at points displaced along the path of the tow.
 3. The process improvement of claim 2 wherein the tow is sprayed with the hydraulic jet both as it enters and leaves the bath. 